Process for decontamination soils polluted with metals

ABSTRACT

The invention relates to a hydrometallurgical process for decontaminating soils 1 which are polluted with metallic elements, comprising a stage of basic leaching I so as to bring about the dissolution of the metallic elements to be removed, a cementation stage II bringing about the precipitation 6 of said elements in metal form, performed by electrochemical exchange with a zinc powder 5 produced by a stage of electrolysis III of the solution 7 originating from the cementation stage. In accordance with the invention a complementary addition of zinc is performed, preferably at the exit of the leaching stage in the form of a leachate 17 of waste from iron and steel manufacture, especially of dust 13 from electrical steel manufacture 15.

FIELD OF THE INVENTION

The invention relates to a process for decontaminating soils pollutedwith metals.

Protection of the environment has become a continuous preoccupation fora number of years and many studies have been conducted to solve theproblems related to pollution.

Pollution of soils with metallic elements, caused in various ways,entails not only the presence of said elements in a concentration thatis too high in relation to the accepted standards, but which can alsoentail a pollution of the aquifer strata and of watercourses.

It is therefore important to have the ability of treating these pollutedsoils so as to decrease this pollution by restoring the concentrationsof metallic elements to levels which conform to the accepted standards.

However, the only decontamination methods available at present are ofpyrolytic type or of hydrometallurgical type using an acidic route, andtherefore they result particularly in the formation of wastes which areimpossible to recycle, requiring storage, or in the release of toxicgases.

PRIOR ART

Nevertheless, a process for hydrometallurgical treatment by a basicroute employing zinc, of the sludge and dust from iron and steelmanufacture is known, for example from Document BE-A-894,733, whichconsists in the combined use of three successive operating stages, andin this order:

leaching of the sludge with soda (therefore a basic medium);

cementation by adding Zn metal to the leaching solution to precipitateand recover the metals which are more electronegative than zinc and tocollect a solution rich in dissolved zinc;

electrolysis of this solution to recover the dissolved zinc by cathodicreduction.

However, this method, designed for the treatment of materials which aresufficiently rich in zinc to begin with (traditionally approximatelyfrom 5 to 50% by weight in the case of steel plants dust) is unsuitablefor the treatment of substances with a low zinc content or even freefrom zinc, as is generally the case with the polluted soils with whichthe invention is primarily concerned.

The same observation applies to the treatment, similar to that outlinedabove, described in Document GB-A-1,568,362.

SUMMARY OF THE INVENTION

The aim of the invention is to arrive at a hydrometallurgical treatmentwith zinc, capable of efficiently decontaminating substances with a lowor zero zinc content, such as polluted soils.

To this end, the subject of the invention is a hydrometallurgicalprocess by a basic route using zinc for decontaminating soils pollutedwith metallic elements, these soils being slightly or not zinc-bearing,comprising a leaching stage so as to bring about the dissolution of themetallic elements to be removed, a cementation stage bringing about thedeposition of said elements in metal form, performed by exchange with azinc powder produced by a stage of electrolysis of the solutionoriginating from the cementation stage, and according to which the zincconcentration of the solution subjected to the electrolysis stage isbrought to a sufficient value (namely approximately at least 8 g/l) tosatisfy the conditions required to obtain zinc powder in theelectrolysis stage, by introducing elemental zinc in a soluble ordissolved form at any stage of the process, complementing the additionof metallic zinc performed in the cementation stage originating from theelectrolysis stage.

In accordance with a preferred embodiment of the invention thecomplementary introduction of elemental zinc is performed at any stagebetween the output from the leaching stage and the input of theelectrolysis stage in the form of a basic leachate of waste from ironand steel manufacture, especially of dust from electrical steelmanufacture. Also preferably, said leachate is added to the mainleachate originating from the treatment of the soils at the exit fromthe leaching stage, before the cementation stage.

One of the secondary characteristics of the invention is that theelectrolysis of the solution is advantageously carried out on amagnesium cathode, preferably with a high current density, which enablesthe zinc to be recovered in the form of ultrafine zinc powder exhibitingproperties which are particularly suitable for its recycling towards thecementation stage of the process according to the invention. However, asa result of its qualities, this ultrafine powder can also be employed inother fields of application and can be optionally marketed in itstotality or partially.

According to one possibility of the invention, a first addition of zincpowder is carried out during the first cementation; the zinc powderproduced subsequently during the electrolysis stage is reintroduced intothe process circuit to carry out the following cementations. Thisadvantageously results in a system for the production and internalconsumption of zinc powders.

In its practical application the process according to the inventionmakes possible a decontamination of soils polluted with metals such as,especially, lead, copper, tin, nickel, arsenic, zinc or cadmium.

In the most frequent cases of polluted soils to be treated, theinvention applies to the treatment of soils which are initially notzinc-bearing; however, it can be applied equally effectively andadvantageously to other applications where the composition to be treateditself contains zinc in any form, for example to slag heaps and otherdumps from iron and steel manufacture.

Thus, the process of the invention can be advantageously applied as aremedy to any pollutions, besides those of natural soils, caused by thepresence of metallic elements, especially in the treatment of spentelectrical batteries or of steel plants dust and for the treatment ofcertain ores.

"Polluted soils" within the meaning of this specification will thereforemean not only natural soils, but any other substance capable ofundergoing a decontamination treatment in order to remove theabove-mentioned undesirable metals therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be understood properly and other aspects andadvantages will emerge from the description which follows, given withreference to the appended plate of drawings, on which:

FIG. 1 gives a basic flow sheet of the process according to theinvention in its general form;

FIG. 2 is a flow sheet illustration similar to FIG. 1, of a preferredembodiment of the process according to the invention.

In the figures the same components are indicated using identicalreferences.

Reference is made firstly to FIG. 1, which is read from left to right:

Leaching stage I

The leaching stage I allows the metallic elements present in thepolluted soil to be treated 1 to be dissolved.

The leaching is preferably carried out in a basic medium created by aninput 2 of NaOH in order to precipitate in the form of insolublehydroxides metals which may be present, such as iron, which will then becapable of being extracted simply by solid-liquid separation, the otherelements going into solution both in basic medium and in acidic medium.

The contaminated soil 1 is first of all passed through a screen, notshown, so as to remove all the gravel and other vegetable debris whichgenerally contain little or no contaminating metals. These debris andgravel are washed to remove the residual soil which may contain thecontaminating metals. It is advantageous for the leaching that the soilshould be screened as finely as possible. Wherever possible, crumblingwould be advantageous in order to improve the kinetics of leaching,which are related to the particle size of the soil.

The soil is next suspended in a soda solution. The metals which are thenpresent go into solution; the iron which may be present precipitates, asalready said, as hydroxide, which will be removed by a subsequentsolid-liquid separation.

The kinetics of leaching of metals in the form of salts, especiallycarbonates and sulfates, or of oxides, are much faster than those ofleaching of metals in metallic form.

Thus, for example, lead reacts according to the following reactions:

    Pb.sup.2+ +4 OH.sup.- →PbO.sub.2.sup.2- +2 H.sub.2 O

    PbO+2 OH.sup.- →PbO.sub.2.sup.2- +H.sub.2 O

In a particularly advantageous manner, aqueous hydrogen peroxide (H₂ O₂)may be injected into the solution so as to oxidize the metals present inmetallic form. For example, in the case of lead the following reactionstake place:

    Pb+H.sub.2 O.sub.2 +2 O.sup.- →PbO.sub.2.sup.2- +2 H.sub.2 O

or

    Pb+1/2 O.sub.2 +2 OH.sup.- →PbO.sub.2.sup.2- +H.sub.2 O

The use of ultrasonics during the leaching stage can advantageouslyimprove the leaching. In fact, ultrasonics permit an increase in thesolid/liquid contact by creating powerful agitation.

In a particularly advantageous manner the use of ultrasonics can alsoentail the formation of aqueous hydrogen peroxide, which is useful forthe oxidation of the polluting elements in metallic form.

Aluminosilicates (clays) may also be present in the soils and they toomay then partially dissolve during the leaching. They can bereprecipitated by injecting lime into the solution.

The presence of salts may also be ascertained and they, too, go intosolution. They will be advantageously removed at the end of treatment byevaporation or by ion exchange resins, if their concentration is toohigh.

A solid/liquid separation is performed next. The soil 3 thusdecontaminated is then washed in basic medium, neutralized, filtered andmay be replaced after a check. The process according to the inventiontherefore allows all the soil treated to be decontaminated.

Cementation stage II

The leachate (or permeate) 4 containing the dissolved metals is thensubjected to a cementation stage II which allows the dissolved metals tobe extracted with the aid of zinc powder by electrochemical exchange.

Thus, for example, in the case of a solution containing lead, thefollowing redox reaction takes place:

    Zn+PbO.sub.2.sup.2- →ZnO.sub.2.sup.2- +Pb

The addition of metallic zinc in powder form 5 is advantageously carriedout so as to bring about a complete reaction between the zinc and thepolluting metals. A new liquid/solid separation is performed next. Themetal cements 6 thus obtained are then washed and filtered off.

According to another special feature of the invention, the metalspresent in the cements 6 can be recycled so that they do not formunusable waste which must be stored as a controlled dump.

The solution 7 originating from the cementation stage, containing zincin the form of zincates, is then subjected to an electrolysis III.

Electrolysis stage III

The electrolysis III according to the invention is carried out inconditions which make it possible to obtain ultrafine zinc powders 8which can be employed entirely or partially 8' for cementation byrecycling. The zinc powder, optionally obtained in excess 8", is in aform which allows it to be marketed for other fields of application,such as especially anticorrosion paints. The basic solution 9 can berecycled to leaching I so as to create a closed-loop liquid circuitbetween the three treatment stages.

The use of a magnesium cathode advantageously makes it possible toobtain a zinc deposit which is not very adherent and in a higher faradayefficiency than with the other cathodes employed during alkalineelectrolyses.

In a particular embodiment of the invention, applied to the treatment ofpolluted soils, the soda concentration of the solution containing thezinc is from 240 to 300 g/l, that is approximately 6N. Thisconcentration makes it possible to attain a maximum conductivity of thesolution and a maximum solubilization of 45 to 50 g/l of zinc.

In this soda solution the zinc is present in the form of zincates ZnO₂²⁻ and dissolves, for example in the leaching stage I, according to thefollowing reaction:

    ZnO+2 OH.sup.- →ZnO.sub.2.sup.2- +H.sub.2 O

In order to obtain the following characteristics in the electrolysisstage III:

Faraday efficiency: >90%

Specific output rate: 1.9 kg/h m²

Energy usage: <4 kWh/kg

the work is done at a solution temperature of 20° to 50° C. andpreferably at 40° C. If 50° C. is exceeded, an excessively adherentmassive deposition and an excessively rapid redissolving of the zincpowder can be observed.

The density of the electrolysis current is high. The most appropriatevalue is a function of the temperature. It generally lies between 10 and30 A/dm², and will be preferably of the order of 20 A/dm².

The solubility limit of zinc oxide in 6N soda is from 45 to 50 g/l. Theconcentration of the zinc solutions which can be treated by the processof the invention lies between 0 and 50 g/l and preferably between 8 and45 g/l. In all cases, if work is done with a concentration of less than8 g/l, there is a corresponding risk that the zinc deposit will separateoff and will then produce short circuits. In addition, the faradayefficiency tends to become lower than 50%.

Since the zinc deposit on the cathode is not very adherent, a pneumaticvibrator may be employed on the cathode, permitting a completeseparation of the deposit, which leaves the cathode clean and smooth.

Thus, the vibrator acts sequentially so as to allow a sufficiently largedeposit to be formed, which separates off proportionately more easilyunder the effect of its own weight.

The formation of a large deposit also entails the advantage of bringingabout a drop in the electrical resistance of the circuit and in theenergy usage, and additionally allows the faraday efficiency to beincreased.

The pneumatic vibrator fitted on the magnesium cathodes isadvantageously employed in cycles of 2 times 5 seconds every 45 min inthe case of zinc concentrations ranging up to 13 g/l and every 15 min inthe case of zinc concentrations of 13 to 8 g/l.

This vibrator preferably has a power equivalent to the vibrator of CFP45type marketed by the French Soci et e Anonyme "Vibration Industrielle"for approximately 5 cathodes of 1 m² ; other equipment may also besuitable.

At the exit from the electrolysis III a washing 10 makes it possible toremove the impurities present in the zinc powder and to destroy the zincagglomerates which may exist.

Since the zinc powder is in a basic medium, it is essential to avoid anycontact with air so as to avoid zinc oxidation.

The washing 10 of the zinc powder is performed with a 6N soda solutionfree from dissolved zinc so as to entrain the complexes of zincate typewhich are present in the solution impregnating the dust, and this makesit possible subsequently to avoid the formation of zinc hydroxide duringthe neutralization.

A passivation 11 is then performed with the aid of sodium dichromate orother passivating agent in order to form a layer of zinc chromate at thesurface of the zinc dust and so to avoid any oxidation and carbonation.

Thus, in the case of sodium dichromate, the reactions governing thepassivation are:

    2 CrO.sub.4.sup.2- +3 Zn+8 H.sub.2 O→2 Cr(OH).sub.3 +3 Zn.sup.2+ +10 OH.sup.-

    3 CrO.sub.4.sup.2- +3 Zn.sup.2+ →3 ZnCrO.sub.4

The washing stage 10 allows for the factors involved in the purity andthe disintegration of the powders.

Thus, a sufficient agitation of the powder permits a removal of thezincates and a complete passivation. A decrease in the particle sizeentails an increase in the specific surface. The mass of the chromate orof another type of passivator must not impair the purity of the powderby an excess which is too large in relation to the quantity needed fortotal passivation. An ultrasonic system is also employed particularlyadvantageously, making possible the disintegration of the powder andalso the fractionation of the fern-like crystals formed.

A decrease in the particle size, an improvement in the contact with thedust and a removal of the zinc hydroxide during the washing with sodaare thus obtained.

The zinc powders obtained exhibit the following characteristics:

purity>95%

mean particle size from 1 to 10 μm

form: fern-like crystals, plates

specific surface from 0.9 to 4 m² /g.

These characteristics allow the zinc powders to be advantageouslyemployed during the cementation stage II of the process of theinvention. For this purpose a separator 12 makes it possible todetermine the quantity 8' to be recycled and the quantity of powder 8"to be recovered, which is washed with demineralized water before dryingin the oven between 80° and 180° C.

Complementary input of zinc A

According to an essential characteristic of the process according to theinvention, an input of elemental zinc is performed as a complement tothe introduction of zinc metal powder in the cementation stage II. Thiscomplementary zinc input can be performed at any time in the process [atA' (leaching), A" (cementation), or (electrolysis)], but in a dissolved(Zn⁺⁺) or soluble (oxide) form. The primary function of this input ofthe elemental zinc is to arrive at a minimum concentration, of the orderof 8 g/l, desired in electrolysis to satisfy the conditions required inthis stage for a stable deposition of zinc powder on the cathode. Thismakes it possible to make up for the deficiency of zinc in the soils tobe treated and which cannot be met solely by the input of metallic zincperformed in the cementation stage II. The primary function of thelatter input is, in fact, to reduce the metals to be recovered as cement6, and it is only in competition with this redox reaction that the zincmetal is subsequently encountered in dissolved form in the electrolyte.

In accordance with a preferred embodiment of the invention, illustratedin FIG. 2, this complementary introduction of zinc is performed indissolved form in the leachate 4 originating from the leaching stage Iand starting with waste from iron and steel manufacture which isnaturally rich in zinc in oxide form, such as the dust from electricalsteel manufacture.

Thus, the dust from iron and steel manufacture 13, originating from theremoval of dust 14 from the fumes from an electrical furnace 15, issubjected to a basic leaching treatment IV, so as, on the one hand, todissolve the Zn by digesting these oxides with soda and, on the otherhand, to precipitate the iron in the form of insoluble hydroxides 16 inorder to enable it to be extracted with a view to a possible recovery.

The basic leachate 17 containing the dissolved zinc is added to the mainleachate 4 from the treatment of the soils upstream of the cementationstage II by means of a mixing valve 18 which makes it possible toregulate the proportion of the leachate 17 in the leachate 4.

There may, in fact, be a disadvantage in introducing this leachate 17already into the leaching stage, because of the property, which somesoils may exhibit, of binding zinc which would be subsequently foundagain in the solid soil fraction 3 returned to the dump, whereas thetreated polluted soil did not contain any, at least to begin with.

This said, the addition of this leachate 17 may be performed at any timebetween the discharge from the leaching stage I and the electrolysisstage III.

If need be, the basic solution 9 originating from the electrolysis canitself be electrolyzed again merely to lower its dissolved zinc contentbefore being recycled into the leaching stage I, if the zinc-adsorptionproperties of the polluted soil were to result in exceeding thestandards permitted by regulations covering releases to the naturalenvironment. The intensity of the electrolysis current may beappreciably lowered in relation to that used in electrolysis I.

It will be noted that sources of complementary zinc other than theleached waste from iron and steel manufacture may be suitable. Forexample, spent electrical batteries nowadays represent a zinc-richdeposit which is quite suitable for this purpose.

The decontamination process according to the invention exhibits a numberof advantages such as its simplicity in use, the absence of secondarypollutions and its selectivity for the metallic elements to be removed.Thus, for example nontoxic iron is not dissolved.

Furthermore, the decontamination process according to the invention mayin some cases allow the treated earth to be replaced back completelywithout having to be dumped, as in the case of other methods.

The decontamination process according to the invention allows theextracted metals to be recycled. The metals 6 thus recovered and thezinc powder 8" produced by the electrolysis can be advantageously resoldso as to be reemployed in various fields of application.

The decontamination process according to the invention thus has theadvantage of being nonpolluting, the only discharge performed being thatof salty water.

The process according to the invention enables zinc to be recovered inthe form of metallic zinc powder which can be employed commercially invarious fields of application, such as especially anticorrosion paints.

It is easy to understand that any traditional hydrometallurgicalprocesses for purifying the solution originating from the cementationstage necessary for treating other pollutants not mentioned as anexample may be employed within the scope of the invention so as to makeit possible to produce a zinc powder usable for the cementation stage ofthe process of the invention, or else marketable.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A particular embodiment will now be described more completely, nolimitation being implied, illustrated by the example hereinafter ofdecontamination of a sample of soil polluted with lead.

A sample of 100 g of soil incorporating a lead content of 1.2% isscreened on a 0.8 mm mesh.

This sample is then subjected to a leaching for a period of 1 h 30 minand at a mean temperature of 95° C. One liter of a soda solutioncontaining from 250 to 300 g/l of NaOH is employed.

An injection of H₂ O₂ at a concentration of 2% per 100 ml is performedfor 30 min after one hour's leaching. If the injection is carried out assoon as the leaching begins, the duration of the latter would beundoubtedly shortened thereby.

Energetic stirring is maintained.

Lime (CaO) is added in a stoichiometric ratio higher than 1 so as toobtain a reprecipitation of the aluminosilicates which may have goneinto solution.

The solid/liquid separation is performed in a centrifuge at 2000 rev/minfor one minute.

The soil is then redissolved in one liter of 6N soda.

The solution is stirred with a mechanical stirrer and then againcentrifuged at 2000 rev/min for one minute.

However, the use of 6N soda solution is not essential, insofar as itsuffices to remain in a basic medium (pH=14) to avoid anyreprecipitation of lead in the form of hydroxide.

The soil recovered after the second centrifuging is neutralized with theaid of hydrochloric acid in one liter of water. The soil is left tosettle for 1 hour and the solution is then filtered. The soil residue isdried for 24 h in an oven at 95° C.

The solution obtained after leaching still contains approximately 1.2g/l of lead and 1 to 3 g/l of silica.

An approximate quantity of 1 liter of basic leachate 17 of dust fromiron and steel manufacture is then added to 40 g of zinc per liter so asto reach a dissolved zinc content of 20 g/l at the time of theelectrolysis, allowing for the addition of dissolved zinc made duringthe cementation.

Powdered zinc metal is next added accurately in a stoichiometric ratioto lead of 1.5 to 2. The zinc powder is advantageously that producedduring the electrolysis.

After one hour of cementation performed with gentle stirring the leadcontent is brought down to less than 70 mg/l.

The electrolysis of the solution containing the zinc is then carried outon a magnesium cathode with a current density of 20 A/dm² at atemperature of 20° C.

A soil lead content which is lower than 300 ppm is thus obtained. Therecovered soil mass is 80 to 90 g and therefore represents a loss ofonly 10 to 20 g.

Naturally, and as is already evident from the above, the invention isnot limited to the particular embodiment which has been described by wayof example, but includes all the alternative forms and equivalents whichcan be read into the definition of the invention given in the attachedclaims.

We claim:
 1. A process for decontaminating soils polluted with metallicelements, comprising the steps of:the polluted soils in a basic mediumto bring about a selective dissolution of metallic elements to beremoved, cementing the leachate originating from the leaching step tobring about the deposition of said elements in metal form, performed byexchange with metallic zinc, electrolyzing the solution originating fromthe cementation step to produce metallic zinc in powder form, andrecycling said powdered metallic zinc in its entirety or partially intothe leachate during the cementation step to satisfy the requirements formetallic zinc in said cementation step, and adjusting the concentrationof zinc in the solution originating from the cementation step to a valueof 8 g/l to satisfy the conditions required to obtain a stabledeposition of zinc powder on a cathode in the electrolysis step byintroducing elemental zinc in a soluble or dissolved form at any step ofthe treatment, complementing the addition of metallic zinc performed inthe cementation step originating from the electrolysis step.
 2. Theprocess for decontaminating polluted soils as claimed in claim 1,wherein the complementary introduction of elemental zinc is performed atthe outlet of the leaching step in the form of a basic leachate of wastefrom iron and steel manufacture.
 3. The process as claimed in claim 2,wherein said waste from iron and steel manufacture is dust fromelectrical steel manufacture.
 4. The process for decontaminatingpolluted soils, as claimed in claim 1, wherein the electrolysis iscarried out with a magnesium electrode forming the cathode.
 5. Theprocess for decontaminating polluted soils as claimed in claim 4,wherein the electrolysis step is performed with a high current densityof between about 10 and 30 A/dm².
 6. The decontamination process asclaimed in claim 4, wherein the cathode in the electrolysis is subjectedto vibrations.
 7. The decontamination process as claimed in claim 4,wherein the electrolysis step is carried out at a temperature of betweenabout 20° to 50° C.
 8. The decontamination process as claimed in claim1, wherein the step of leaching in a basic medium is carried out with asoda solution with a strength of 240 g/l to 300 g/l.
 9. Thedecontamination process as claimed in claim 1, wherein an injection ofaqueous hydrogen peroxide is performed during the leaching step.
 10. Thedecontamination process as claimed in claim 1, wherein the leaching stepis performed at a temperature higher than or equal to 90° C.
 11. Thedecontamination process as claimed in claim 1, wherein the leaching stepis performed in the presence of ultrasound.
 12. The decontaminationprocess as claimed in claim 1, wherein during the step of cementationwith zinc powder, the latter is added in a stoichiometric ratio of 1 to2 to the metallic element.